# Event logging and telemetry

JupyterHub can be configured to record structured events from a running server using Jupyter's [Telemetry System]. The types of events that JupyterHub emits are defined by [JSON schemas] listed at the bottom of this page.

## How to emit events

Event logging is handled by its `Eventlog` object. This leverages Python's standing [logging] library to emit, filter, and collect event data.

To begin recording events, you'll need to set two configurations:

> 1. `handlers`: tells the EventLog _where_ to route your events. This trait is a list of Python logging handlers that route events to the event log file.
> 2. `allows_schemas`: tells the EventLog _which_ events should be recorded. No events are emitted by default; all recorded events must be listed here.

Here's a basic example:

```
import logging

c.EventLog.handlers = [
    logging.FileHandler('event.log'),
]

c.EventLog.allowed_schemas = [
    'hub.jupyter.org/server-action'
]
```

The output is a file, `"event.log"`, with events recorded as JSON data.

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## Event schemas

```{toctree}
:maxdepth: 2

server-actions
```

[json schemas]: https://json-schema.org/
[logging]: https://docs.python.org/3/library/logging.html
[telemetry system]: https://github.com/jupyter/telemetry